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fc59a04061
- approach by Hacker's Delight with wrapping subtraction - performance expected to be similar to addo - tests with all relevant combinations of min,max with -1,0,+1 and all combinations of sequences +-1,2,4..,max
38 lines
1.3 KiB
Zig
38 lines
1.3 KiB
Zig
const builtin = @import("builtin");
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// subo - subtract overflow
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// * return a-%b.
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// * return if a-b overflows => 1 else => 0
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// - suboXi4_generic as default
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inline fn suboXi4_generic(comptime ST: type, a: ST, b: ST, overflow: *c_int) ST {
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@setRuntimeSafety(builtin.is_test);
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overflow.* = 0;
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var sum: ST = a -% b;
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// Hackers Delight: section Overflow Detection, subsection Signed Add/Subtract
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// Let sum = a -% b == a - b - carry == wraparound subtraction.
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// Overflow in a-b-carry occurs, iff a and b have opposite signs
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// and the sign of a-b-carry is opposite of a (or equivalently same as b).
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// Faster routine: res = (a ^ b) & (sum ^ a)
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// Slower routine: res = (sum^a) & ~(sum^b)
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// Overflow occured, iff (res < 0)
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if (((a ^ b) & (sum ^ a)) < 0)
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overflow.* = 1;
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return sum;
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}
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pub fn __subosi4(a: i32, b: i32, overflow: *c_int) callconv(.C) i32 {
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return suboXi4_generic(i32, a, b, overflow);
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}
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pub fn __subodi4(a: i64, b: i64, overflow: *c_int) callconv(.C) i64 {
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return suboXi4_generic(i64, a, b, overflow);
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}
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pub fn __suboti4(a: i128, b: i128, overflow: *c_int) callconv(.C) i128 {
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return suboXi4_generic(i128, a, b, overflow);
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}
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test {
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_ = @import("subosi4_test.zig");
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_ = @import("subodi4_test.zig");
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_ = @import("suboti4_test.zig");
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}
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